Including Parents and Relatives in the Will was later abrogated
Allah says;
كُتِبَ عَلَيْكُمْ إِذَا حَضَرَ أَحَدَكُمُ الْمَوْتُ إِن تَرَكَ خَيْرًا
الْوَصِيَّةُ لِلْوَالِدَيْنِ وَالاقْرَبِينَ بِالْمَعْرُوفِ حَقًّا عَلَى الْمُتَّقِينَ
It is prescribed for you, when death approaches any of you, if he leaves wealth, that he makes a bequest to parents and next of kin, according to reasonable manners. (This is) a duty upon Al-Muttaqin (the pious).
This Ayah contains the command to include parents and relatives in the will, which was obligatory, according to the most correct view, before the Ayah about inheritance was revealed.
When the Ayah of inheritance was revealed, this Ayah was abrogated, so fixed shares of the inheritance for deserving recipients were legislated by Allah. Therefore, deserving inheritors take their fixed inheritance without the need to be included in the will or to be reminded of the favor of the inherited person.
For this reason we see the Hadith narrated in the Sunan and other books that Amr bin Kharijah said;
I heard Allah's Messenger saying in a speech;
إِنَّ اللهَ قَدْ أَعْطَى كُلَّ ذِي حَقَ حَقَّهُ فَلَ وَصِيَّــةَ لِوَارِث
Allah has given each heir his fixed share. So there is no will for a deserving heir.
Imam Ahmad recorded that Muhammad bin Sirin said;
Ibn Abbas recited Surah Al-Baqarah until he reached the Ayah;
إِن تَرَكَ خَيْرًا الْوَصِيَّةُ لِلْوَالِدَيْنِ وَالاقْرَبِينَ
(...if he leaves wealth, that he makes a bequest to parents and next of kin), he then said,
"This Ayah was abrogated."
This was recorded by Sa`id bin Mansur and Al-Hakim in his Mustadrak.
Al-Hakim Said, "It is Sahih according to their criteria (Al-Bukhari and Muslim)."
Ibn Abu Hatim reported that Ibn Abbas said that Allah's statement;
الْوَصِيَّةُ لِلْوَالِدَيْنِ وَالاقْرَبِينَ
(a bequest to parents and next of kin),
was abrogated by the Ayah;
لِّلرِّجَالِ نَصيِبٌ مِّمَّا تَرَكَ الْوَلِدَنِ وَالاٌّقْرَبُونَ وَلِلنِّسَأءِ نَصِيبٌ مِّمَّا تَرَكَ الْوَلِدَنِ وَالاٌّقْرَبُونَ مِمَّا قَلَّ مِنْهُ أَوْ كَثُرَ نَصِيباً مَّفْرُوضاً
There is a share for men and a share for women from what is left by parents and those nearest related, whether the property be small or large ـ a legal share. (4;7)
Ibn Abu Hatim then said,
"It was reported from Ibn Umar, Abu Musa, Sa`id bin Musayyib, Al-Hasan, Mujahid, Ata Sa`id bin Jubayr, Muhammad bin Sirin, Ikrimah, Zayd bin Aslam and Ar-Rabi bin Anas. Qatadah, As-Suddi, Muqatil bin Hayyan, Tawus, Ibrahim An-Nakhai, Shurayh, Ad-Dahhak and Az-Zuhri said that;
this Ayah (2;180 above) was abrogated by the Ayah about the inheritors (4;7)."
The Will for the Relatives that do not qualify as Inheritors
It is recommended that the remaining relatives who do not have a designated fixed share of the inheritance, be willed up to a third, due to the general meaning of the Ayah about the will.
It is recorded in the Sahihayn that Ibn Umar said that;
Allah's Messenger said;
مَا حَقُّ امْرِىءٍ مُسْلِمٍ لَهُ شَيْءٌ يُوصِي فِيه يَبِيتُ لَيْلَتَيْنِ إلاَّ وَوَصِيَّتُهُ مَكْتُوبَةٌ عِنْدَه
It is not permissible for any Muslim who has something to will to stay for two nights without having his last will and testament written and kept ready with him.
Ibn Umar commented,
"Ever since I heard this statement from Allah's Messenger, no night has passed, but my will is kept ready with me."
There are many other Ayat and Hadiths ordering kindness and generosity to one's relatives.
The Will should observe Justice
The will should be fair, in that one designates a part of the inheritance to his relatives without committing injustice against his qualified inheritors and without extravagance or stinginess.
It is recorded in the Sahihayn that Sa`d bin Abu Waqqas said,
"O Allah's Messenger! I have some money and only a daughter inherits from me, should I will all my remaining property (to others)?"
He said, "No."
Sa`d said, "Then may I will half of it?"
He said, "No."
Sa`d said, "One-third?"
He said, "Yes, one-third, yet even one-third is too much. It is better for you to leave your inheritors wealthy than to leave them poor, begging from others."
Al-Bukhari mentioned in his Sahih that Ibn Abbas said,
"I recommend that people reduce the proportion of what they bequeath by will to a fourth (of the whole legacy) rather than a third, for Allah's Messenger said;
الثُّلُثُ وَالثُّلُثُ كَثِير
One-third, yet even one-third is too much."
Allah's statement